Highly variable response of hard coral taxa to successive coral bleaching events (2019-2020) and rising ocean temperatures in Northeast Peninsular Malaysia

Abstract

Based on current greenhouse gas emission trajectories, Malaysian coral reefs are predicted to experience severe annual coral bleaching events by 2043, imminently threatening the survival of Malaysian coral reefs within this century. However, there is no field data on how Malaysian coral reefs respond to successive sequences of coral bleaching. Numerous scleractinian taxa have shown the ability to acclimatize to thermal stress events after previous exposure to heat disturbances. Nonetheless, thermal tolerance and acclimatization potentials might corroborate with accelerating warming rates and increasing frequencies of thermal stress anomalies, necessitating repeated field studies at reef scale to investigate thermal tolerance and acclimatization of scleractinian taxa. Here, we studied two successive thermal stress events during the 2019 El Niño Southern Oscillation (ENSO) and during the onset of the La Niña Oscillation in 2020. We recorded the bleaching susceptibility of scleractinian taxa to document bleaching trajectories across fine temporal and environmental gradients in Northeast Peninsular Malaysia. In addition, we analyzed historic temperature trends to demonstrate rapid warming rates (0.17°C per decade) and high return frequencies of thermal stress anomalies. Despite high maximum temperatures in both years (31.07°C and 31.74°C, respectively), accumulated thermal stress was relatively low during the bleaching episodes (Degree Heating Weeks 1.05°C-weeks and 0.61°C-weeks, respectively) and marginally varied across reef scales (0.94°C-weeks, 0.76°C-weeks, 0.48°C-weeks in 2020), suggesting a widespread thermal sensitivity of most scleractinian taxa (55.21% and 26.63% bleaching incidence in 2019 and 2020, respectively). However, significant discrepancies between satellite and in-situ temperature data were found (0.63°C; SD±0.26). Bleaching susceptibility was highly taxon-specific and contrasted historical bleaching patterns (e.g., Acropora and Montipora showed high thermal tolerance). In 2020, successive heat disturbance moderately increased bleaching susceptibility of three taxa (Galaxea, Leptastrea and Platygyra) despite lower heat stress, while Heliopora was highly susceptible in both years. Bleaching analysis of taxa on biophysical reef scales revealed significant difference across depth, wind sites (e.g., leeward and windward), and the combined interactions of wind and depth (e.g., leeward shallow) on bleaching response were significant for numerous taxa. Findings suggest thermal acclimatization of fast-growing taxa, whereby successive bleaching events and accelerating warming rates selectively pressure scleractinian assemblages.

Using relative return‐on‐effort scoring to evaluate a novel coral nursery in Malaysia

“…In addition, our study specifically measured growth and survival of nursery corals across two distinctive seasons: the warm summer season (March to October), and the colder northeast monsoon season (November to February). In areas with this seasonal pattern heat stress induced coral bleaching typically occurs between April and September (Guest et al 2012; Szereday & Amri 2021). During the first 215 days of our study (March to October), locally recorded sea surface temperatures were the second highest on average since 1985, resulting in moderate to widespread coral bleaching (Szereday & Amri 2021).…”

“…In areas with this seasonal pattern heat stress induced coral bleaching typically occurs between April and September (Guest et al 2012; Szereday & Amri 2021). During the first 215 days of our study (March to October), locally recorded sea surface temperatures were the second highest on average since 1985, resulting in moderate to widespread coral bleaching (Szereday & Amri 2021). Despite high temperatures and subsequent heat stress, the three nonacroporid species ( E. horrida , H. rigida , and P. cylindrica ) grew significantly faster during the warmer season.…”